The goal of incorporating tetrahedral titanium(IV) in classical zeolite structures has been actively pursued for more than a decade. Such materials include tetrahedral titanium incorporated into ZSM-5 analogs such as TS-1, substitution into ZSM-11 analogs such as TS-2, substitution into zeolite Beta and, more recently, incorporation into MCM-41. This interest results from the unique catalytic properties of Ti(IV) sites in molecular sieve configurations.
Commercially, the hydroxylation of phenol to catechol and hydroquinone is practiced using TS-1. Other reactions which have demonstrated promise include oximation of cyclohexanone as well as olefin epoxidation, especially the conversion of propylene to propylene oxide. A potentially important commercial reaction involving the rearrangement of oxime to lactam has also been reported.
The known examples of tetrahedral Ti (IV) in zeolite structures have several elements in common. First, the amount of Ti incorporation is relatively low, typically 2-3 wt % or less. Second, zeolites such as TS-1, Ti-Beta, Ti-MCM-41 etc., are silica rich; i.e., there are only low levels of framework aluminum in these zeolites. This means that the ion-exchange capacity and the potential acid concentration, (the number of potential acid sites), in these zeolites is low. The low level of ion-exchange sites in zeolites with tetrahedral Ti incorporation means that the ion-exchange properties, the adsorptive properties and potential catalytic applications are also restricted.
In contrast, the materials of this invention combine relatively high levels of tetrahedral Ti incorporation with high levels of ion-exchange capacity. This high ion-exchange capacity arises from framework charge neutralization associated with octahedrally coordinated framework titanium atoms. Each octahedral titanium atoms results in two negative framework charges which must be neutralized with cations or other appropriate species.
ETS-10 molecular sieve (U.S. Pat. No. 4,853,202) is a crystalline structure consisting of silica chains linked to octahedral titania chains. As such, it contains both tetrahedral and octahedral framework sites.
Reference is made to the following:
S. M. Kuznicki and K. A. Thrush; U.S. Pat. No. 5,244,650 and U.S. Pat. No. 5,208,006. PA1 M. W. Anderson, A. Philippou, Z. Lin, A. Ferreira and J. Rocha; Angew. Chem. Int. Engl., 34, 1003 (1995). PA1 Rocha, Z. Lin, A. Ferreira and M. W. Anderson; J. Chem. Soc. Chem. Commun., 867 (1995). PA1 ratio of solid/liquid in the molecular sieve slurry; PA1 the pH of the slurry; PA1 the temperature of the slurry during ATH addition; PA1 the rate of ATH addition; PA1 the amount of ATH added relative to the sieve; PA1 time and temperature of treatment after ATH addition is complete; PA1 degree of washing.
In U.S. Pat. No. 3,329,481 the synthesis of charge bearing titanium silicates using a peroxy reagent during synthesis is disclosed. Distinctions between the resulting "titanium zeolites" and ETS-10 molecular sieve are set forth in detail in U.S. Pat. No. 5,244,650.
U.S. Pat. No. 5,208,006, commonly assigned, discloses and claims a host of crystalline titanium molecular sieves of the ETS-10 type having at least one octahedrally coordinated site comprising titanium and at least tetrahedrally coordinated silicon. The tetrahedral sites may include, in addition to silicon, any one of a host of metals, one of which may be titanium. The terms "octahedral coordination" and "tetrahedral coordination" are defined in U.S. Pat. No. 5,208,006 at col. 19. The teachings of U.S. Pat. No. 5,208,006 are incorporated herein in full by cross-reference.
Unlike other atoms, the chemical environment in a conventional ETS-10 synthesis mixture forces essentially all of the titanium into octahedral coordination to form titanium silicate chains so that direct synthesis, especially the controlled direct synthesis of mixed octahedral/tetrahedral sites is not achieved using conventional procedures.
Procedures for decreasing the overall Si/Ti ratio of titanium silicate sieves are not disclosed.